Flammable gas concentration measurements are made in a number of safety-critical situations. One such flammable gas is natural gas, which typically comprises mainly methane, plus higher hydrocarbons, inert gases and trace components. Natural gas detectors are needed for a number of applications including response to public reported gas escapes and for continuous monitoring of plant/equipment using permanently installed detectors. They are required to measure the gas concentration as a percentage of the lower explosion limit (LEL) of the gas mixture, this being an important safety parameter.
The concentration of a flammable gas component, such as methane, in a gas mixture can, theoretically, be measured by infrared spectroscopy, using a filter having a peak transmission wavelength equal to one of the wavelengths of absorption by methane, e.g. at 3.32 μm. The filter and the light source together define a selected range of wavelengths over which the spectral measurement of the gas mixture is made, giving a degree of selectivity for individual gas species. The measured concentration can be converted into % LEL to give an indication of the safety of the gas mixture. However, gas mixtures from natural sources will usually contain unknown proportions of other flammable components, such as hydrocarbons including ethane, propane and butane, while in some cases such components are deliberately added. The presence of these additional flammable components disturbs the accuracy of the % LEL measurement, by absorbing infrared radiation to a degree which is out of proportion with their influence on LEL, relative to methane.